1. Field of the Invention
The invention generally relates to displacement measuring apparatuses. More particularly, the invention relates to a displacement measuring apparatus for use in measurements of linear and rotational displacements.
2. Description of the Related Art
Displacement measuring apparatuses are generally known as the apparatus for implementing precise measurements of linear or rotational displacements, and other similar displacements, which are implemented with a scale and a detector head. The measuring apparatuses have been widely used, for example, in a packaging equipment of electronic components, which generally requires precise positioning control of materials on conveyance, and in a measuring equipment of the size of parts and components. The detection system of the measuring apparatuses may be divided broadly into two types, magnetic and optical.
As to the known displacement measuring apparatus of magnetic type, for example, one approach has been disclosed in Japanese Unexamined Patent Application Publication No. 2002-267493, and a magnetic encoder is therein described concerning a magnetic encoder utilized for detecting the position and controlling movable materials.
The magnetic encoder disclosed in the Application Publication No. 2002-267493 includes at least a magnetic scale including a magnetic pole pattern, a detection element which is provided facing the magnetic pole pattern and includes plural detection patterns for detecting the magnetic field, a holder for holding the detection element, and a processing circuit which is connected to the detection element and configured to implement rectangular processing of input signals obtained from the detection element, characterized in that the distance between one of the plural detection patterns and the magnetic scale is approximately equal to that between other detection patterns and the magnetic scale.
According to the magnetic encoder disclosed in the Application Publication No. 2002-267493, the amount of displacement of the detection element can be measured relative to the magnetic scale by detecting the magnetic pole pattern of the magnetic scale by means of the detection element.
As to the known displacement measuring apparatus of optical type, for example, there disclosed in Japanese Unexamined Patent Application Publication No. 2006-177876 is a displacement measuring apparatus of laser interference type.
The displacement measuring apparatus disclosed in the Application Publication No. 2006-177876 is provided which includes at least a scale including a diffraction grating, and a detector head unit which is provided to be movable along the direction of measuring axis relative to the scale and configured to emit coherent light toward the scale as well as to receive the light diffracted by the scale. In the displacement measuring apparatus, the detector head unit includes a light source for emitting coherent light, a demultiplexing means for demultiplexing the coherent light emitted from the light source into two light beams, optical elements which are provided for each of the light beams outputted from the demultiplexing means and configured to reflect the light beams and subsequently to render the light beams be incident to the scale as incident beams, and retroreflective means which is provided for each of diffracted light beams generated by diffracting the two light beams previously rendered incident to the scale by means of the diffraction grating and are configured to retroreflect the diffracted light beams and subsequently to render the diffracted light beams be incident as recursive light beams. In the displacement measuring apparatus, the incident beams and the recursive light beams are rendered incident perpendicular to the grooves of the diffraction grating and that the angle between the incident beams and the normal vector of the scale is larger than the angle between the recursive light beams and the normal vector of the scale.
According to the displacement measuring apparatus disclosed in the Application Publication No. 2006-177876, by detecting the light diffracted by the diffraction grating which is formed on the scale, by the optical detection unit, the amount of displacement can be measured relative to the scale.